A. Field of the Invention
This invention relates to offset lithography and printing generally. It relates more specifically to method and apparatus for imaging lithographic and other planographic plates.
B. Description of the Related Art
There are a variety of known ways to print hard copy in black and white and in color. The traditional techniques include letterpress printing, rotogravure printing and offset printing. These conventional printing processes produce high quality copies. However, when only a limited number of copies are required, the copies are relatively expensive. In the case of letterpress and gravure printing, the major expense results from the fact that the image has to be cut or etched into the plate using expensive photographic masking and chemical etching techniques.
Plates are also required in offset lithography. However, the plates are in the form of mats or films which are relatively inexpensive to make. The image is present on the plate or mat as hydrophilic and hydrophobic (and oleophilic, or ink-receptive) surface areas. In wet lithography, water and then ink are applied to the surface of the plate. Water tends to adhere to the hydrophilic or water-receptive areas of the plate creating a thin film of water there which does not accept ink. The ink does adhere to the hydrophobic areas of the plate and those inked areas, usually corresponding to the printed areas of the original document, are transferred to a relatively soft blanket cylinder and, from there, to the paper or other recording medium brought into contact with the surface of the blanket cylinder by an impression cylinder.
In dry lithography, application of a fountain or dampening solution to the plate prior to inking is unnecessary. Instead, the non-image material of dry plates is itself sufficiently ink-repellent that ink simply fails to adhere to such material.
Most conventional offset plates are produced photographically. In a typical negative-working, subtractive process, the original document is photographed to produce a photographic negative. This negative, or "mask", is placed on an aluminum plate having a water-receptive oxide surface that is coated with a photopolymer. Upon being exposed to light through the negative, the areas of the coating that received light (corresponding to the dark or printed areas of the original) cure to a durable oleophilic or ink-receptive state. The plate is then subjected to a developing process which removes the noncured areas of the coating that did not receive light (corresponding to the light or background areas of the original). The resultant plate now carries a positive or direct image of the original document.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required, each of which is usually made photographically as aforesaid. In addition to preparing the appropriate plates for the different colors, the plates must be mounted properly on the print cylinders in the press and the angular positions of the cylinders coordinated so that the color components printed by the different cylinders will be in register on the printed copies.
The development of lasers has simplified the production of lithographic plates to some extent. Instead of applying the original image photographically to the photoresist-coated printing plate as above, an original document or picture is scanned line-by-line by an optical scanner which develops therefrom strings of picture signals, one for each color. These signals are then used to control a laser plotter that writes on and thus exposes the photoresist coating on the lithographic plate to cure the coating in those areas which receive light. That plate is then developed in the usual way by removing the unexposed areas of the coating to create a direct image on the plate for that color. Thus, it is still necessary to chemically etch each plate in order to create an image on that plate.
A number of designers have attempted to automate the platemaking process by etching digitally stored image data onto a blank lithography plate. One well-known method of accomplishing this is known as electro-erosion. The type of plate suitable for imaging in this fashion has an eleophilic plastic substrate, e.g., Mylar plastic film, having a thin coating of aluminum metal with an overcoating of conductive graphite; the latter acts as a lubricant to minimize scratching of the aluminum coating. A stylus electrode in contact with the graphite surface coating is caused to move across the surface of the plate and is pulsed in accordance with incoming picture signals. The resultant current flow between the electrode and the thin metal coating is, by design, large enough to erode away the thin metal coating and the overlying conductive graphite surface coating, thereby exposing the underlying ink-receptive plastic substrate on the areas of the plate corresponding to the printed portions of the original document.
This method of making lithographic plates suffers from the disadvantage that the described electro-erosion process only works on plates whose conductive surface coatings are very thin; furthermore, the stylus electrode which contacts the surface of the plate still sometimes scratches the plate. This degrades the image being written onto the plate because the scratches constitute inadvertent or unwanted image areas on the plate that print similarly unwanted marks on the copies.
An alternative to the electro-erosion process is described in U.S. Pat. No. 4,718,340. This reference describes use of spark-discharge apparatus that do not make contact with the plate, thereby avoiding the above-noted problem of surface scratching. Because the disclosed apparatus operate at relatively low power levels, the plates discussed in this reference all have hydrophilic metal substrates coated with an oleophilic surface layer, the latter being ablated during the imaging process. This places significant limitations on the ability to design plates that will perform both durably and effectively. For example, to be suitable for use with the disclosed apparatus, the oleophilic surface layer must be fragile enough to decompose upon exposure to relatively low-energy sparks, a characteristic that would also result in limited on-press durability. Furthermore, the approach described in this patent would not be suitable for production of typical dry plates, which feature oleophobic silicone surface coatings that could not usefully be employed in conjunction with a hydrophilic substrate, in addition to being too resilient for removal at low power levels.
Another example of non-contact platemaking is described in published European Patent Application EP 0167352, which involves a method for generating a latent image on a blank plate using low-current electrical discharges. These discharges do not remove or otherwise alter the physical characteristics of the substrate. Rather, the apparatus appears to produce the same chemical changes in the plate-surface material that would conventionally be produced by exposure to actinic radiation, as described above. Once again, the need for chemical response to the low-power discharges imposes significant limitations on the plate constructions that can be imaged with this type of apparatus.
Other designers have attempted to use more powerful lasers to write images on blank plates. However, the use of such lasers for this purpose has not been entirely satisfactory because the photoresist coating on the plate must be compatible with the particular laser, a requirement that limits the choice of coating materials. Also, the pulsing frequencies of some lasers used for this purpose are so low as to render the time required to produce a halftone image on the plate unacceptably long.
There have also been some attempts to use scanning E-beam apparatus to etch away the surface coatings on plates used for printing. However, such machines are very expensive. In addition, they require that the workpiece, i.e., the plate, be maintained in a complete vacuum, making such apparatus impractical for day-to-day use in a printing facility.
We are also aware of a press system that images a lithographic plate while the plate is actually mounted on the print cylinder in the press. The cylindrical surface of the plate, treated to render it either oleophilic or hydrophilic, is written on by an ink jetter arranged to scan over the surface of the plate. The ink jetter is controlled so as to deposit on the plate surface a thermoplastic image-forming resin or material that has a desired affinity for the printing ink being used to print the copies. For example, the image-forming material may be attractive to the printing ink so that the ink adheres to the plate in the areas thereof where the image-forming material is present but is repelled by the "wash" used in the press to prevent inking of the background areas of the image on the plate.
While that prior system may be satisfactory for some applications, it is not always possible to provide thermoplastic image-forming material that is suitable for jetting and also has the desired affinity (philic or phobic) for all of the inks commonly used for making lithographic copies. Also, ink jet printers are generally unable to produce small enough ink dots to allow the production of smooth continuous tones on the printed copies, i.e. the resolution is not high enough.
Thus, despite all of the aforementioned efforts to improve different aspects of lithographic plate production and offset printing, significant performance and operational limitations remain.